Heat absorption balancing system for a steam generator having a primary steam circuit and a reheating steam circuit



June 1, 1965 c. STROHMEYER, JR 3,186,1 75 HEAT ABSORPTION BALANCINGSYSTEM FOR A STEAM GENERATOR HAVING A PRIMARY STEAM CIRCUIT AND AREHEATING STEAM CIRCUIT Filed Jan. 14, 1963 4 Sheets-Sheet l aqa RADIANTssc'nou 45 Fig.!.

INVENTOR. Charles Strohmeyer,Jr.

his ATTORNEY June 1965 c. STROHMEYER, JR 3,186,175

HEAT ABSORPTION BALANCING SYSTEM FOR A STEAM GENERATOR HAVING A PRIMARYSTEAM CIRCUIT AND A REHEATING STEAM CIRCUIT Filed Jan. 14, 1963 4Sheets-Sheet 2 CONVECTION SECTION 47 RADIANT SECTION 45 INVENTOR.

I Charles Strohmeyer, Jr. F lg. 2. BY

his ATTORNEY June 1, 1965 c. STROHMEYER, JR 3,186,175 HEAT ABSORPTIONBALANCING SYSTEM FOR A STEAM GENERATOR HAVING A PRIMARY STEAM CIRCUITAND A REX-[EATING STEAM CIRCUIT Flled Jan. 14, 1965 4 Sheets-Sheet 3 K70TURBINE g) I83 I84 I85 2 5|. 186 I87 I90 -Is4 wk V ll- ITo mi 1-H6 97 QI72 I4I H68 !74 I I9 -Ios I42 /I75 I76 /I95 E O E I09 E E p :09 P I 9 PP '09 P?- I2I I23 I05 I07 I96 :3 I22 I we /I27 /I78 II 53 IIo :45 I28 2!at M A R FUEL FLOW AIR FLOW l-REHEAT 2 REHEAT I- 0:1 TEMPERATURETEMPERATURE F Ig. 3.

INVENTOR. Charles Sfrohmeyer, Jr. BY

his ATTORNEY June 1,1965 c. STROHMEYER, JR 3,186,175

HEAT ABSORPTIO A STEAM N BALANCING SYSTEM FOR GENERATOR HAVING A PRIMARYSTEAM CIRCUIT AND A REHEATING STEAM CIRCUIT 4 Sheets-Sheet 4 Filed Jan.14, 1963 INVENTOR. Charles Strohmeyer, Jr.

United States Patent 3,186,175 HEAT ABSORPTEQN BALANQING SYSTEM FQR ASTEAM GENERATQR HAVHQG A PRIMARY STEAM CIRQUIT AND A REHEATING STEAM(IIRCUIT (Shartes Strohmeyer, Jn, Wyomissing, Pa., assignor to GilbertAssociates, lino, Reading, Pa. Filed Jan. 14, 1963, Ser. No. 251,358 2tilaims. (Cl. 60-73) This invention relates to devices and systems forimproving the heat absorption balance among the variouscircuits of asteam generator throughout the operating load range when deliveringsteam to a turbine driver for powering an electric generator, and wheresteam from intermediate turbine stages is returned to the steamgenerator for reheating.

In such a plant, high pressure superheated steam from the steamgenera-tor is admitted to a high pressure turbine. At some intermediatepoint in the turbine steam flow path, steam is exhausted to a conduitwhich conveys the steam to the steam generator where it is passedthrough reheating heat absorption conduits located in the heatingmedium. After reheating, the steam again enters the turbine to performfurther work as the steam expands. The turbine may have a single steamreheating stage in which case, after performing its useful work, thesteam exhausts from the turbine at low pressure to a condenser.

Steam may be reheated in more than one stage. In such case, the pressureof each succeeding reheating stage is lower than the upstream reheatingstage pressure.

Steam exhausted to the condenser is condensed and pumped back to thesteam generator feedwater inlet through a regenerative feedwater heatingcycle wherein steam is extracted from the turbine steam flow path atintermediate pressures to successively raise the temperature of thefeedwater before it enters the steam generator.

The feedwater passes through a preheating section, a generating sectionand a super-heating section. The high pressure circuit from theeconomizer inlet to the superheater outlet is called the primary steamcircuit. Steam returned from the turbine to the steam generator forreheating and return to the turbine is called the 1st reheat steamcircuit and the 2nd reheat steam cincuit.

Where there is one stage of reheating, the primary steam circuitoperates in parallel with the 1st reheat steam circuit. The primary and1st reheat outlet steam temperatures should be maintained at valueswhich will produce maximum plant efficiency throughout as large aportion of the load range as is practical to produce maximum operatingeconomy. The steam genera-tor is normally designed to produce specifiedoutlet steam temperature for each of the primary and reheat circuits ata minimum control load. For loads above this point, some additionalmeans for control is necessary to properly proportion heat input to eachcircuit to produce the specified steam temperature. For loads below thecontrol load, steam temperature will depart from the specified valuewithout means for further correction. Known systems for controllingsteam temperatures of both circuits throughout at least a portion of theload range are expensive and are sometimes wasteful of power for drivingauxiliary equipment as is the case where gas recirculation fans are usedto proportion heat input to various steam generator components. Wherethere are two stages of reheating, the steam temperature control problembecomes more complex and more expensive to achieve both with respect tocapital investment in equipment and cost of plant operation.

An object of this invention is to provide an efficient and economicalmeans for the aforementioned steam generator of the reheat type wherebyheat may be transferred from a reheat steam circuit to the primary steamcircuit in varying relative amounts throughout at least a portion of theload range to balance heat absorptions in the circuits'exchanging heatto maintain specified outlet steam temperatures in both circuits.

' A more specific object of this invention is to provide said means fora steam generator having a primary circuit and a single steam reheatingstage.

A further specific object of this invention is to provide said means fora steam generator having a primary c rcult and two steam reheatingstages.

A still further specific object of this invention is to provide a systemfor control of feedwater flow, heat input and outlet steam temperaturesfor said steam generators where they are of the once-through type.

A still further specific object of this invention is to provide aneconomical and efiicient heat exchanger for exchanging heat betweenreheat and primary steam circuits.

Other objects and advantages of this invention will become more apparentfrom a study of the following description taken with the accompanyingdrawings wherein:

FIG. 1 is a schematic diagram of a steam generator and steam turbinecycle having one stage of steam reheating, and embodying the principlesof the present inventionj FIG. 2 is diagram of a steam generator andsteam turbine cycle having two stages of steam reheating, and embodyingthe principles of the present invention;

:FIG. 3 is a diagram of a feedwater flow, heat input and outlet steamtemperature control system for FIGS. 1 and 2; and,

FIGS. 4a. and 4b are an arrangement for a heat exchanger to transferheat from the reheat steam to the primary steam.

General description of present invention In a steam-electric generatingplant consisting of a steam generator, a steam turbine of the reheattype and an electric generator driven by the steam turbine, coordinationof the steam generator and turbine with respect to steam temperatures isdesirable to obtain the maximumoverall plant efficiency.

FIG. 1 illustrates such a plant. The steam generator 1 shown in dash anddot outline is of the once-through type. {High pressure superheatedsteam from steam generator 1 flows through conduit 2., through fiowcontrol valve 3 to the high pressure turbine 4. After releasing energypreforming work, the steam in the high pressure turbine is reduced inpressure and temperature and exhausts through conduit 5 tolow'temperature reheater 6 and high temperature reheater 7. After thesteam is elevated in temperature, it is returned through conduit 8 duit159 to pump 2i] which raises the pressure of the fluid to the Workingpressure of the steam generator. Pump 20 discharges through conduit 21to high pressure heating system 22 which receives steam fromintermediate turbine stages through conduit 23. High pressure heatingsystem 22 discharges through conduit 24 to the steam generator Ipreheating section'or economizer 25. Turbines 4, 9 and it drive electricgenerator through shaft 56.

In steam generator 1, the feedwater entering econo- I) mizer flowsthrough the economizer and discharges through conduit 26 to the inletheader 27 which feeds fluid to the furnace waterwalls 29. Flow passesthrough waterwalls 29 and discharges to outlet header 30. Flow passesfrom outlet header 30 through conduit 31 to the inlet header 32 whichfeeds fluid to the convection pass wall enclosure 33. Convection passwall enclosure 33 discharges to outlet header 34 and through conduit 35to primary superheater 36. From the primary superheater, fluid passesthrough conduit 37 to the radiant superheater 40. The discharge of theradiant superheater 40 passes through conduit 41 to the finalsuperheater 42. The final superheater 42 discharges to conduit 2.

The steam generator 1 has combustion furnace 28. Conduits 43 supply fuelto burners 44. Combustion air is supplied to burners 44 (not shown). Thecombustion air and the fuel from burners 44 are fired in the furnace 28.Heat released in furnace 28 is transmitted to heat absorbing circuits29, 7 and 4t) mostly as radiant heat. This section of the steamgenerator is denoted as the radiant section 45. The hot gases fromcombustion pass and are cooled over heat absorbing circuits 33, 42, 6,36 and 25 and exit from the steam generator at 46. This latter portionof the steam generator is denoted as the convection section 47. Heat istransferred to the circuits 33, 42, 6, 36 and 25, mostly by convection.

In order to maintain specified steam temperatures in conduits 2 and 8,the circuits 25, 29, 33, 36, 40 and 42 must be arranged in properbalance with circuits 6 and 7 so each group will receive itsproportionate and balanced 7 share of heat input from the combustionproducts in furnace 28. If the balanced condition exists at one designcontrol load of steam generator 1, then at other loads, this balancedcondition is disturbed. As load is decreased, the ratio of heatabsorption increases in the radiant section 45 and decreases in theconvection section 47. The reverse is true as load is increased.Therefore,

design steam temperatures cannot be maintained over a wide load rangeunless the proper division of radiant and convection heat input to theprimary and reheat steam circuits can be maintained throughout the sameload range.

A once-through steam generator can be constructed so that there is nofixed division point between steam generating and superheating dutiesbetween economizer 25 circuit including conduit 5 reheaters 6 and 7,their interconnecting conduits and, conduit 8. Spray water increases themass flow of reheat steam and in effect bypasses fluid around the highpressure turbine 4. This I lowers overall plant efliciency.

Other known means of reheat steam temperature control include multi-passarrangements for gas flow wherein dampers regulate gas flow over thevarious heat absorption conduits to balance steam temperatures. This isexpensive and wasteful of effective heat transfer surface; also, controlresponse is slow. Gas recirculation,

where exit gas at 46 is returned to furnace 28 through 'gas ducts usinga fan to provide the required pumping head, is also used to controlabsorption distribution between the radiant and convection sections. Gasrecirculation increases convection section absorption and decreasesradiant section absorption. Gas recirculation is expensive to installand consumes considerable horsepower to pump and recirculate hot gasesthrough the flow path. Gas recirculation is not shown on FIG. 1.

Another method is the use of tilting burners, where 1- the direction ofburner flame is raised or lowered in a vertical furnace so as toincrease heat input to the convection section as the burners are turnedin the direction of the convection section. While this may be used as ameans of balancing temperatures, availability of such burners islimited.

A drum type steam generator having recirculation circuits in thegenerating section has similar characteristics for primary and reheatsteam temperature control. Since the evaporation and superheating dutiesare rigidly divided by the steam drum, artificial means must be employedto upset the natural balance. Spray water can be injected into thesuperheater to lower primary steam temperature without loss of plantefficiency. This transfers some of the evaporating duty to thesuperheater. In such case, the superheater is designed for specifiedoutlet steam temperature at the partial control load. For higher loads,the ratio of superheater absorption increases, requiring spray waterinjection to lower outlet steam temperatures to the specified value.Firing rate is adjusted to maintain steam pressure, and spray water inthe superheater is regulated to control steam temperature.

Therefore, the present invention overcomes the past difficulties withrespect to controlling steam temperatures at the outlet of two paralleland separate fluid flow paths of a steam generator, and where both flowpaths receive heat from a common heat generating source as furnace 2S,and where one fluid path is operated at a substantially higher pressurethan the other, said one fluid flow path discharging to the steam inletof a prime mover, said other fluid flow path receiving steam from anintermediate stage of said prime mover for reheating and return to thesame said prime mover.

More specifically, the present invention relates to a new and novelsystem and devices to maintain constant outlet steam temperatures forboth fluid flow paths for at least a portion of the load range withoutthe need for alteration of the relative position of the firing means orof the gas flow path through recirculation means and without the use ofspray water injection into either of the flow paths as the basic elementof proportioning heat transfer to the fluid flow paths. This isaccomplished by distributing the heat absorption surface of each fluidflow path in the steam generator in a Way which will produce anapproximate balance between flow paths, and providing means to exchangeheat between flow paths externally from the common heat generatingsource flow path when the balance in temperatures between fluid flowpaths is disturbed. Thus, the common heat generation source combustionrate can be regulated to control steam temperature at the outlet of onefluid flow path and the outlet steam temperature of the second fluidflow path can be regarded by exchanging heat with the first fluid flowpath.

FIG. 1 shows one of many possible arrangement combinations for achievingthe broad structural objectives. Heat exchanger 48 is used to exchangeheat from the reheat circuits 6 with the generating section fluid flowfrom economizer 25. The low temperature reheater 6 discharges throughconduit 49 to heat exchanger 48, wherein the reheated steam passes overand gives up heat to tubes 5-3. The cooled steam exits through conduit51 to high temperature reheater '7 and from thence to turbine 9 throughconduit 8. A portion of the fluid from conduit 26 passes through conduit52 to heat exchange tubes in heat exchanger 48. Valve 53 regulates theamount of flow through conduit 52. Tubes 50 discharge to conduit 54,conduit 31 and to inlet header 32 in series.

Flow through conduit 52, tubes 59 and conduit 51 bypasses furnacewater-wall circuits 29. This provides hydraulic head necessary to passfluid through conduit 52, tubes 50 and conduit 54'. The small reducedflow through circuit 29 does not significantly reduce the heat transferto these circuits. The total heat to header 30 remains s,1se,175

the same. The lesser mass flow will have a higher enthalpy. The enthalpyof fluid from economizer 25 is also increased as it passes through tubes59.

When fluid is passed through tubes 56, the combined enthalpy of thefluid is increased as it enters header 32. Heat exchange rate in heater48 is regulated by control of flow through valve 53.

The reheater is constructed of both convection and radiant heat transfersurfaces. For full load operation, cooling of steam in heat exchanger 43is necessary to maintain outlet'steam temperature from high temperaturereheater 7 at specified value (say 1000 F.). Flow through valve 53 iscontrolled to maintain specified temperature in conduit 8.

As load is decreased, less heat is absorbed in low ten perature reheater6. This lowers steam temperature in conduit 3. Flow through valve 53 isreduced to main.- tain specified steam temperature in conduit 8. Firingrate of burners 44 is controlled to maintain specified temperature ofthe superheater outlet steam in conduit 2. (say 1000 F).

Thus, radiant and convection heat absorption can be balanced between thetwo flow streams by adjustment of firing rate in burners 44 and flowrate through valve 53. Relatively small flows will be required throughvalve 53. Their order of magnitude may range from 5 to 20 percent ofrated primary steam how. The sh ll of heater 48 is designed to Withstandthe low pressure fluid from the exhaust of turbine 4.

it should be noted that when there is excessive absorption in circuits 6and 7, heat transfer to tube 5% reduces the furnace heat input requiredto produce specified steam temperature in conduit 2. This amplifies theeffectiveness of heat exchanger 43 for controlling steam temperature inconduit 8.

In the case of a double reheat steam-electric generating plant, as shownin FIG. 2, there is the problem of balancing three steam temperaturesexiting from the steam generator. The same numbers have been used fordenoting identical parts in FIGS. 1 and 2. The element numbers for thereheat circuit of FIG. 1 have been applied to the second reheat circuitin FIG. 2. In PEG. 2 the heat exchanger 43 is installed in the secondreheat circuit.

In FIG. 2, steam exiting from superheater d2 flows through conduit 2 tovery high pressure turbine 4. Turbine 4 exhausts through conduit 57 toconvection reheater 58, through conduit 59 to radiant reheater 6h.Reheater 6t discharges to conduit d1 which transports the steam to highpressure turbine 62. Turbine 6?. discharges through conduit 5 toreheater 6, through conduit 49 to heat exchanger 48, from heat exchanger.3 to conduit 51 and reheat-er 7, back to turbine 9 through conduit 8.The remainder of the cycle is the same as for FIG. 1. Valves 63 and 64control the flow of reheat steam to turbines 62 and 9 in times of upsetor emergency.

In accordance with the above discussion of steam temperature control forFIG. 1, primary steam temperature for FTG. 2 in conduit 2 may becontrolled by means of variations in firing rate for the once-throughtype of steam generator illustrated. The 2nd reheat outlet steamtemperature in conduit 8 may be controlled by varying flow through valve53 as described above for FIG. 1. The outlet steam temperatures inconduits 2 and 8 are in effect bias-ed one with the other by means ofheat exchanger 43.

There remains the problem of controlling the lst reheat outlet steamtemperature in conduit 61. This may be accomplished by one of severalmethods presently available. In all cases the primary and 2nd reheatsteam temperatures will be biased as a unit with the 1st reheat steamtemperature. The biasing method will depend upon the particular meansavailable to the individual steam generator manufacturer.

devices and structures of this invention.

Biasing may be achieved by means of damper controls (not shown). In suchcase, part of reheater 53 is arranged parallelly with superheater 36 inthe gas flow path so that the gases from combustion furnace 28 willenter sections 58 and 36 at the same relative temperatures. Absorptionbetween sections 58 and 36 is proportioned by means of dampers whichregulate ratio of gas flow between the two parallel sections, The 1streheat outlet steam temperature is controlled by means of the damperaction, the primary steam temperature is controlled by means ofregulating firing rate of burners 44 and 2nd reheat steam temperature iscontrolled by means of regulating flow through valve 53.

Biasing may be achieved by means of gas recirculation. In such case, hotexit gas from convection section 47 is drawn off from conduit 46 throughconduit 65. Fan 66 provides the head required to recirculate gas throughfurnace 28. As gas recirculation is increased, a greater percentage ofthe heat absorption is transferred to the convection section 47. The 1stheat steam temperature is controlled by means of regulating gasrecirculation flow, primary steam temperature is controlled by means ofregulating firing rate of burners 44 and 2nd reheat steam temperature iscontrolled by means of regulating flow through valve 53.

Biasing may be achieved by means of varying elevation of the combustionzone. In such case, the fuel and air flow from burners 44 are divertedup or down through angle 68. This in turn affects the absorption in thefurnace 28 and gas temperatures entering convection section 47. The 1streheat steam temperature is controlled by means of regulating the angleof flame discharge from burners 44, the primary steam temperature iscontrolled by regulating firing rate and the 2nd reheat steamtemperature is controlled by regulating flow through valve 53.

The arrangement of heat transfer surfaces shown on FIGS. 1 and 2 arerepresentative. Other arrangements are possible which will employ thegeneral principles, For example, Where gas recirculation is employed asa principle means to bias the primary and 2nd reheat steam temperatureswith the 1st reheat steam temperature, radiant sections 60 and 49 wouldpreferably be minimized or eliminated and the reheating surface ofsections 53 and 42 would be increased correspondingly.

It is also evident that heat in both the 1st and 2nd reheat circuitcould be exchanged with the primary circuit. In such case, flow fromconduit 26 would be divided between two heat exchangers arranged inparallel to that shown for 48, each heat exchanger receiving anddischarging flow from one of the two reheat circuits. It is also evidentthat heat exchanger 48 could be used in conjunction with the 1st reheat,using supplemental means as described above for control of the secondreheat steam temperature.

Therefore, the present invention as it relates to heat exchanger 48offers great design flexibility. The heat exchanger 48 arrangement maybe adapted to suit the many commercial types of steam generators incombination with a wide variety of steam turbine cycles and steamconditions for primary and reheat steam.

FIG. 3 is an integrated feedwater flow, fuel flow, air

flow and steam temperature control system for the arrangement shown onFIG. 2. The same numbers are used for denoting identical parts in bothFIGS. 2 and 3. The control system shown on FIG. 3 is electricallyintegrated and is of the O-10 volt direct current operational amplifiertype, such as manufactured by the Hagan Chemicals and Controls, Inc.,Hagan Center, Pittsburgh 30, Pa.

The load demand signal is the required output of the steam generator.This may be in the form of steam flow as measured from primary steamflow in conduit 2, or in the form of steam flow as measured from turbine4, 1st stage pressure, or in the form of load requirement 7 establishedby an external source as a central dispatch ofiice, or in the form ofgenerator 55 load in kilowatts, or in the form of AC frequency output ofgenerator 55, or in combinations thereof or other equivalent means. Thespecific form of the load demand signal is not a part of this inventionother than that it exists.

Turbine 69 is a composite of turbines 4, 62, 9 and 11, as shown in FIG.2-. In FIG. 3, the circuit 76 transmits the load demand signal andconverts it to a O to 10 volts DC. current in circuit 72. In all casesvoltage increases the load signal and converts it to a to volts DC.current in circuit 72. In all cases voltage increases proportionally toincreases in required output of the controlled variables, such asfeedwater fiow, fuel flow,

etc. Increases in the load demand signal increase the feedwater flow,fuel and air flow controlled variables and vice versa for decreases inthe load demand signal.

Fluid at 73 is drawn to steam pressure transmitter 74 through conduit'75. Transmitter 74 converts fluid pressure to electrical voltage inoutput circuit 76. Circuit 76 feeds to reset amplifier 77 where thesteam pressure signal is compared to set point. Set point adjuster 78establishes the value to which steam pressure shall be controlled.Increases in steam pressure above set point decrease the feedwater flow,fuel and air kow controlled variables through the integral action untilthe pressure returns to set point and vice versa for decreases in steampressure. The integral action accumulation persists after pressure hasreturned to the set point. Circuit 79 transmits the output of resetamplifier 77 to amplifier 80 where the circuit 79 signal is added to thecircuit 72 signal. Derivative action from rate of voltage change incircuit 72 is incorporated in amplifier 80. The derivative action isonly sustained as long as a rate of change in circuit 72 voltagepersists, after which the derivative action is cancelled after a shortinterval of time. Means are provided i amplifier 81) for adjusting thereference level of output voltage in circuit 81. Circuit 81 feeds toamplifiers 82 and 83.

Thermocouple 84 in primary steam conduit 2 in rc- 1 sponse totemperature generates a potential in circuits 85 and 86 when referencedto cold junctions in transmitter 87. Transmitter 87 sends out a variablevoltage in circuit 88 proportional to changes in steam temperature inconduit 2. Circuit 88 feeds to amplifiers 39 and 90. The circuit 88voltage is compared with set point voltage 91 to 89. Increases in steamtemperature above set point decrease the output of amplifier 39 incircuit 92 and vice versa for decreases in steam temperature.

The reset action of amplifier 89 is similar to that of amplifier 77.Derivative amplifier 9% is responsive to voltage changes in circuit 88.Increasing temperature from 87 decreases the voltage output in circuit93 for the duration of the change and vice versa for decreasingtemperature. The signals of circuits 92 and 93 are totalized inamplifier 94. The output circuit 95 feeds to amplifiers 82 and 83.

In amplifier 82, the signals from circuits 81 and 95 are added together.Means are provided for adjusting the gain of each. Means are alsoprovided for varying the reference level of the output voltage incircuit 96. Circuit 96 feed to reset amplifier 97 which is similar toamplifier 77. Means are provided to measure fuel flow in conduit 43(shown in FIG. 2). Where oil is the fuel, orifice 98 is provided withpressure taps 99 and 101i. Taps 99 and 100 feed to transmitter 101 shownin FIG. 3 which converts proportional increases in fuel flow toproportional increases in control voltage in circuit 102. Circuit 102feeds to reset amplifier 97. Amplifier 98 increases or decreases theoutput voltage in circuit 103 until the signals in circuits 96 and 102are equal. If the actual flow as measured in circuit 102 is below therequired flow as measured in circuit 96, the voltage of the 193 circuitwill increase to open valve 108 until a balance is achieved betweencircuits 96 and 102. Circuit 103 feeds to electric-pneumatic converter104 where proportional increases in the control voltage in circuit 103are converted to proportional increases in pneumatic air pressure inconduit 1115. Conduit 105 feeds to valve positioner 106 which isprovided with cam means to produce a given valve 108 opening for eachgiven air pressure in conduit 1115. Positioner 11% supplies power air tovalve operator 107 to change the position of valve 168. Conduit 109supplies control air to converter 104. Conduit 1111 supplies power airto positioner 196 and valve operator 107.

Feedwater flow is measured across nozzle 111 in conduit 24. Pressureconduits 112 and 113 across nozzle 111 connect to transmitter 114.Transmitter 114 converts differential pressure across nozzle 11 toelectric voltage in a way so that proportional increases in flowincrease electric voltage proportionally in circuit 115. Circuit 115feeds to reset amplifier 116 which functions the same as 97 to balanceactual feedwater flow in circuit 115 with required feedwater flow incircuit 117 by opening and closing feedwater flow control valve 118.Circuit 119 corresponds to circuit 103, electric-pneumatic converter 120to 194, conduit 121 to 165, positioner 122 to 1116, valve operator 123to 107. Amplifier 83 subtracts the circuit 95 signal from the circuit 81signal. Means are provided for adjusting the gain of each. Means arealso provided for referencing the level of the output voltage in circuit117. Also, the output gain from amplifier 83 may be biased with respectto the output gain from amplifier 82.

In FIG. 2, air is supplied to burners 44 through air supply duct 124.Damper 125 controls the flow of air. Lever arm 126 connected to damperoperator 127 through connecting link 123 positions damper 125. Air flowis measured through air foil 129. Taps 139 and 131 sense differentialpressure through the air foil 129. Conduits 130 and 131 connect totransmitter 132 shown in FIG. 3 which converts proportional increases anair flow as measured by differential pressure in conduits 130 and 131 toproportional changes in electric voltage in circuit 133. Circuit 133feeds to optional ratio control 134 which ratios the air flow signalfrom 132 to amplifier 136 so as to maintain the proper fuel-air ratio inthe burners 44 as measured in an oxygen meter taking samples from theflue gas in conduit 46 and sending the oxygen quantity measurementsignal to circuit 135 after passing through a reset amplifier with setpoint (not shown). Ratio control 134 feeds to reset amplifier 136through circuit 137 (134 could be omitted for purposes ofsimplification). Fuel flow measurement in circuit 102 is compared withair flow measurement in circuit 137 and the output signal of 136 incircuit 138 is varied until there is the proper balance as described foramplifier 97. Thus, air flow is made to follow fuel flow with a fixedratio between the two.

Circuit 138 feeds to amplifier 139. The signal in circuit 96 which isthe base signal and which reflects rapid change in the load demandsignal is added to the circuit 138 signal in amplifier 139. The gain ofthe signal in the circuit 96 may be adjusted for calibration purposes.Also, means are provided for varying the reference level of the outputvoltage in conduit 149. Circuit 140 feeds to bias amplifier 141 and biassetter 142. The bias amplifier permits adjustment of air flow ratio withrespect to fuel flow. Amplifier 141 feeds through circuit 142 toelectric-pneumatic converted 143 which is similar to 104.

The pneumatic signal from 143 passes through conduit 144 to position145. Positioner 145 feeds air to power piston operator 127 to drive itto the correct position. A cam and lever associated with the pistonoperator 127 .and positioner 145 controls the supply of air to thepiston from positioner 145. The piston will assume a predeterminedposition for each control air pressure variation in conduit 144. Thus,piston position may be characterized as desired with air pressure inconduit 144. Piston op- 9 erator 127 drives air flow control damper 125through connecting link 128.

Thermocouple 146 is located in the 1st reheat outlet steam conduit'61.The circuits from 146 to circuit 166 are the same as from 84 to circuit95. Element 149 corresponds to 87, 162 to 91, 16b to 89, 161 to 90 and155 to 94. When lst reheat steam temperature rises above the set pointin 162, voltage decreases in circuit 166, decreasing gas recirculationflow and vice versa for steam temperatures below set point. Load demandtrans mitter '71 sends a signal to amplifier 167 through circuit 72which increases gas recirculation flow proportional with load decrease.Maximum gas recirculation is required at minimum control load. Circuit166 adds to the circuit 72 signal described above. Means are providedfor referencing the output of 167 in circuit 163.

Gas recirculation flow is measured across orifice 169 in conduit 67shown in FIG. 2. Pressure taps 170 and 171 sense the differentialpressure across orifice 169. In FIG. 3, taps 170 and 171 connect to flowtransmitter 172 which functions similar to element 132.

The voltage increase in circuit 173 is proportional to gas flow increasein conduit 67. Actual gas flow measured in circuit 173 is balancedagainst required gas flow in circuit 168 and the output signal incircuit 175 is increased or decreased until circuits 1'73 and 168 arebalanced. Increased voltage in circuit 175 increases gas recirculationfl-ow. Electric-pneumatic converter 176 is similar to 143 and 194. Theaction of positioner 177 and piston operator 178 is similar to elements145 and 127. Piston movement is transmitted through link 1811 to leverarm 181 which rotates damper 182 in the gas recirculation conduit 65shown in FIG. 2.

Thermocouple 183 is located in the second reheat outlet steam conduit 8.The circuits from 183 to circuit 1'94 are similar to those from 84 tocircuit Q5.

Element 1% corresponds to =87, 188 to 89, 189 to 90, 190 to 91 and 193to 94. The difference is that when 2nd reheat steam temperature risesabove the set point in 190, voltage increases in circuit 194, increasingopening of valve 53, passing more primary circuit flow through the heatexchanger 48. Since heat input to the primary steam circuit iscorrespondingly increased, firing rate should be decreased accordingly.The reverse happens when 2nd reheat steam temperature falls below theset point. Circuit 19 feeds to electric-pneumatic converter 195 which issimilar to 194. The control action from converter 195 to valve 53 issimilar to that from 1& to valve 108. Positioner 1% corresponds to 106,valve operator 197 to 107, valve 53 to valve 108. Circuit 19 1 alsofeeds to amplifier 82. The gain of the circuit 194 signal may be variedbefore it is subtracted from the signal-s from circuits 81 and 95 inamplifier 82 Thus, as 2nd reheat temperature rises, firing rate isdecreased and heat transfer from the 2nd reheat steam circuit to theprimary steam circuit is increased.

It is also obvious that there is flexibility with respect to thearrangement of the control system. Alternatively, the thermocouples 8 3and 183 could be interchanged with respect to their locations in thesteam piping so that 2nd reheat steam temperature would control fuel andair flow direct and primary steam temperature would control valve 53. Insuch case, the circuit -95 connection to amplifier 83 would be omitted.

In order to adapt FIG. 3 to the FIG. 1 arrangement, elements 57, 53, 60,61 and 63 are omitted. The system from thermocouple 146 to pistonoperator 17S and connecting links including the circuit '72 connectionto amplifier 167 are not required. The 2nd reheat control circuit wouldbe used for the 1st and only reheat steam circuit.

In the case where the steam generator has a 1st and 2nd reheat steamcircuit and a heat exchanger 43 is provided for each reheat circuit toexchange heat with the primary steam circuit as described above, thecontrol for regulating flow through each heat exchanger would be thesame as 10 that shown in FIG. 3. Two systems would be required tooperate in parallel, each having duplicate components to those shown inFIG. 3.

FIGS. 4a and 4b show an arrangement for heat exchanger 48 which can beused with FIGS. 1 and 2. Reheat steam enters the heater she'll throughconduits 49 and flows back and forth across tubes 59. Bafiies 198 directthe how of reheat steam from the core 230 out toward the shell and'bafiies 199 direct the flow of reheat steam from the shell to the core.The reheat steam exits through conduits 51. The core diameter, bafflelocations and shell diameter are arranged to produce reheat steamvelocities in the vicinity of ft. per second across the tubes 59 at fullload flows. The tubes 50 are arranged concentr-ically around the shell,as shown in FIG. 4b. Flow through the tubes 56 is multipass. Primarycircuit fluid from conduit 52 discharges to feeder tubes 201 which supply the multipass tube circuits 59. The mu'ltipass arrangement for eachtube 50 circuit is contained within heater shell 48. Gnly the inlet andoutlet of each multipass tube circuit protrudes through the heatershell. The connec tions 201a are broken off to show a clearer picture ofthe arrangement. They too supply the individual multipass tube circuits5%. The outlet of the multipass tube circuits connect to discharge tubes2M2. Tubes 292:: are broken ed for purposes of clarity. Tube 202 and202a connect to conduit 54. Tubes 50 are finned on the exterior surfaceto increase the heat'transfer surface on the reheat steam :side. Primarycircuit fluid iiows through the inside of the tubes. The heater shell 48has an inner jacket 263 in the vicinity of the reheat steam inlet whichis separated from the external shell sufficiently to pass cooling steamfrom conduits 2 34 between the jacket and the shell. Cooling steam maybe obtained through conduits 204 and flow control valves 205' fromconduit 5 of FIGS. 1 and 2.

' The heat transfer in heater 48 is very eflicient compared with heattransfer to the superheaters in the steam generator. The transfer ratefor the heater 48 can be as high as B.t.u. per square foot of externalheating surfiace per hour per degree of log mean temperaturedifferential compared with about 6 to 10 B.t.u. to the same base forheat transfer which occurs in the superhe-aters in the steam generatorconvection section 4'7. To minimize the total length of the highpressure tube 50 circuits, the tubes are finned on the outside to raisethe heat transfer rate from the reheat steam (containing superheat) tothe tube metal so as to be more nearly equal to the transfer rate fromthe tube metal to the internal fluid. Thus, the high pressure materialis minimized.

Thus, heat absorbed in low pressure reheater 6 can be effectively andefficiently transferred to the primary steam circuit in heater 4-8. Thisreduces the heat absorption duty required for the primary circuit tosupport full load specified temperature operation which offsets to agreat extent the added cost for the heat exchanger 48. Nor- .mally somesurplus reheating surface as is employed in 1 FIGS. 1 and 2 is requiredfor the reheat circuits to pro vide somecontrol range for reheat steamtemperature below design rating. The efficiency gains as a result ofelimination of reheat spray water and reduced cost of control equipment,as a result of the present invention is evident.

In a 450,000 kilowatt steam electric generating plant having 3500p.s.i.g. and 1000 F. primary steam and two reheats at 1025 and 1050 F.respectively, the heat exchanger 48 inlet reheat steam would be about380 p.s.i.g. and 900 F. reducing the steam temperature about 75 F.through the unit before the reheat steam exits through conduits 51. Theprimary fluid temperature entering would be about 600 F., and would exitat about 700 F. Reheat steam flow through heater 48 would be about2,240,000 lbs/hr. and primary fluid flow would be about 300,000 lbs/hr.The finned tube surface required would be about 5,000 sq. ft.

FIGS. 1 and 2 are not intended to be limiting with respect to locationof heat exchanger 48. For example,

''i. exchanger 43 could be located in the reheat conduit where the steamtemperature was sufliciently high (as 800 F.) to produce economical rateof heat transfer er square foot of transfer surface. Precise location ofheat exchanger 48 and connecting conduits will depend upon the specificrequirements of each application. Also, the conduit 52 can connect tothe primary circuit upstream of preheater 25 instead of to conduit 26without significant change in the performance of the system. For thesame reason, conduit 54 can discharge to some downstream location otherthan conduit 31, as shown in FIGS. 1 and 2.

Thus, it will be seen that I have provided eflicient systems for a steamgenerator having a primary steam circuit and a reheating steam circuit,which systems Will improve the heat absorption balance throughout atleast a portion of the load range for maintaining specified outlet steamtemperatures in both circuits; furthermore, I have provided a novelapparatus and system for accomplishing the above, whereby heat may betransferred from a reheat steam circuit to the primary steam circuit invarying relative amounts to balance heat absorptions in the circuitsexchanging heat; also, I have provided a novel apparatus and system forcontrolling outlet steam temperatures for a steam generator having aprimary circuit and one reheating stage; also, I have provided a novelapparatus and system for controlling outlet steam temperatures for asteam generator having a primary circuit and two reheating stages; also,I have provided a novel system to integrate steam temperature controlwith control of steam generator feedwater flow and firing rate;furthermore, I have provided a novel heat exchange apparatus forexchanging heat in the reheat steam circuit with the primary steamcircuit.

I claim:

1. A steam-electric generating plant and under variable load conditionscomprising a steam generator having a high pressure primary circuit forflowing fluid therethrough, said primary circuit having heat absorptionconduits located serially in preheating, generating and superheatingsections of said steam generator, a steam consumer turbine for drivingelectric generating means, conduit means for interconnecting said steamgenerator primary circuit outlet and said steam consumer turbine, saidsteam generator including reheating heat absorption conduits, additionalconduit means for conveying steam, after partial use in said steamconsumer turbine from said steam consumer turbine to said reheating heatabsorption conduits and for returning said partially consumed steam,after reheating, to said steam consumer turbine, a combustion furnacefor generating hot products of combustion, and duct means arrangedserially for conveying said hot products of combustion over and amongsaid steam generator heat absorption conduits, a bypass conduit, saidbypass conduit connected to the primary circuit at both ends andbypassing at least a portion of said preheating and generating sectionheat absorption conduits for establishing hydraulic head to flow aportion of the primary circuit fluid through said bypass conduit from anupstream to downstream point of the primary circuit, an auxiliary heatexchange means located in said additional conduit means and adapted toexchange heat with fluid flowing through said bypass conduit, flowcontrol means in said bypass conduit, said reheating heat absorptionconduits and said auxiliary heat exchange means being adapted forregenerative heating of the primary circuit fluid, said regenerativeheating per unit of primary circuit flow con tinuously increasingthroughout at least a portion of the upper load range as said plant loadincreases and at essentially constant primary circuit and reheating heatabsorption conduit fluid temperatures at outlet points.

2. A steam-electric generating plant and under variable load conditionscomprising a steam generator having a high pressure primary circuit forflowing fluid therethrough, said primary circuit having heat absorptionconduits located serially in preheating, generating and superheatingsections of said steam generator, a steam consumer turbine for drivingelectric generating means, conduit means for interconnecting said steamgenerator primary circuit outlet and said steam consumer turbine, saidsteam generator including reheating heat absorption conduits dividedinto low temperature and high temperature components, interconnectingconduit means for flowing fluid serially from said low temperature tosaid high temperature components, additional conduit means for conveyingsteam, after partial use in said steam consumer turbine, from said steamconsumer turbine to the inlet of said low temperature component and forreturning said partially consumed steam after reheating in said lowtemperature and high temperature components to said steam consumerturbine, a combustion furnace for generating hot products of combustion,and duct means arranged serially for conveying said hot products ofcombustion over and among said steam generator heat absorption conduits,a bypass conduit, said bypass conduit connected to the primary circuitat both ends and bypassing at least a portion of said preheating andgenerating section heat absorption conduits for establishing hydraulichead to flow a portion of the primary circuit fluid through said bypassconduit from an upstream to downstream point, an auxiliary heat exchangemeans located in said interconnecting conduit means and adapted toexchange heat with fluid flowing through said bypass conduit, flowcontrol means in said bypass conduit, said reheating heat absorptionconduits and said auxiliary heat exchange means being adapted forregenerative type heating of the primary circuit fluid, saidregenerative type heating per unit of primary circuit flow continuouslyincreasing throughout at least a portion of the upper load range assaidplant load increases and at essentially constant primary circuit andreheating heat absorption conduit fluid temperatures at outlet points.

References ited by the Examiner UNITED STATES PATENTS 3,111,936 11/63Brunner 122-479 FOREIGN PATENTS 1,225,893 2/60 France.

137,548 4/30 Switzerland.

JULIUS E. WEST, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

1. A STEAM-ELECTRIC GENERATING PLANT AND UNDER VARIABLE LOAD CONDITIONSCOMPRISING A STEAM GENERATOR HAVING A HIGH PRESSURE PRIMARY CIRCUIT FORFLOWING FLUID THERETHROUGH, SAID PRIMARY CIRCUIT HAVING HEAT ABSORPTIONCONDUITS LOCATED SERIALLY IN PREHEATING, GENERATING AND SUPERHEATINGSECTIONS OF SAID STEAM GENERATOR, A STEAM CONSUMER TURBINE FOR DRIVINGELECTRIC GENERATING MEANS, CONDUIT MEANS FOR INTERCONNECTING SAID STEAMGENERATOR PRIMARY CIRCUIT OUTLET AND SAID STEAM CONSUMER TURBINE, SAIDSTEAM GENERATOR INCLUDING REHEATING HEAT ABSORPTION CONDUITS, ADDITIONALCONDUIT MEANS FOR CONVEYING STEAM, AFTER PARTIAL USE IN SAID STEAMCONSUMER TURBINE FROM SAID STEAM CONSUMER TURBINE TO SAID REHEATING HEATABSORPTION CONDUIT AND FOR RETURNING SAID PARTIALLY CONSUMED STEAM,AFTER REHEATING, TO SAID STEAM CONSUMER TURBINE, A COMBUSTION FURNACEFOR GENERATING HOT PRODUCTS OF COMBUSTION, AND DUCT MEANS ARRANGEDSERISLLY FROM CONVEYING SAID HOT PRODUCTS OF COMBUSTION OVER AND AMONGSAID STEAM GENERATOR HEAT ABSORPTION CONDUITS, A BYPASS CONDUIT, SAIDBYPASS CONDUIT CONNECTED TO THE PRIMARY CIRCUIT AT BOTH ENDS ANDBYPASSING AT LEAST A PORTION OF SAID PREHEATING AND GENERATING SECTIONHEAT ABSORPTION CONDUITS FOR ESTABLISHING HYDRAULIC HEAD TO FLOW APORTION OF THE PRIMARY CIRCUIT FLUID THROUGH SAID BYPASS CONDUIT FROM ANUPSTREAM TO DOWNSTREAM POINT OF THE PRIMARY CIRCUIT, AN AUXILIARY HEATEXCHANGE MEANS LOCATED IN SAID ADDITIONAL CONDUIT MEANS AND ADAPTED TOEXCHANGE HEAT WITH FLUID FLOWING THROUGH SAID BYPASS CONDUIT FLOWCONTROL MANS IN SAID